Inductive loads, such as a coil of a line contactor switch for example, which are operated at a low-voltage switching device with DC control or control via a rectifier (AC/DC), only drop out very slowly after removal of a control supply voltage despite a free-wheeling circuit provided in the low-voltage switching device to reduce a shutdown overvoltage caused in such a case by the inductive load. In the worst case the result is what is referred to as a 2-step drop out, meaning for example that contacts switched in a main current path that are switched with the inductive load, are in contact with each other for a brief period without any spring force. The contacts can then easily become welded together or only have a short electrical service life overall.
Even if the inductive load is activated electronically, the free-wheeling circuit must be designed as a controlled or self-controlled circuit in order to ensure the fastest possible reduction of the magnetic energy stored in the inductive load when the inductive load is shut down.
It is generally known that this problem can be resolved by way of a diode or a Zener diode within the free-wheeling circuit.
The high power losses which occur permanently in such cases are a disadvantage with such solutions.
One variant in such solutions is to switch on and shut down the free-wheeling circuit in a controlled manner. In normal operation the free-wheeling circuit is shut down so that the power losses no longer occur permanently. To this end coil activation electronics evaluate switching thresholds and, depending on whether said thresholds are exceeded or not reached, the free-wheeling circuit is switched on or shut down, for example via an optocoupler.
Corresponding coil activation electronics are known from the document DE 195 19 757 C2 for example.